Moisture at Walls
2012-01-12
There are several potential causes of a moisture at an interior wall. These include gaps or cracks at the exterior, wind driven rain through soffits, leaking plumbing pipes, vapor diffusion, etc. These are discussed further below. Groundwater seepage below an exterior wall usually manifests at a floor or lower wall, and is discussed near the middle of the page found here. Leaks at windows and doors also cause moisture damage at walls, usually adjacent to or below the leaking window or door. Moisture may manifest as blistered paint, staining, mold, etc. Direct penetration of rainwater sometimes occur through soffit vents and gaps and cracks at the exterior of a home. Soffits are usually perforated or vented for attic ventilation and wind driven rain may penetrate through or around the soffits, resulting in stains at adjacent interior upper walls. Rain may also penetrate gaps or cracks at the exterior of a home. Larger gaps or cracks obviously have more potential to leak. This condition may also be related to or exacerbated by sprinkler heads directed towards the affected area. A leak in a plumbing pipe may also be the cause of a wall stain. Plumbing supply pipes are often placed in interior walls. Plumbing supply pipes are pressurized and a very large and continuous quantity of water is often released when they leak such that the cause can hardly be mistaken. This is not always the case, as slower leaks at fittings or 'pinholes' also occur. Vapor drive (or diffusion) may also cause a moisture problem at a wall. Water vapor moves from areas of higher temperature and vapor pressure towards areas of lower temperature and vapor pressure. In northern climates, a vapor retarder is typically placed at the inside face of exterior walls. During cold winter months, the retarder is intended to prevent water vapor (either from air movement or vapor drive) from entering the wall. This is because vapor that enters the wall passes through a thermal gradient which exists across the depth of the wall. Water vapor will condense into liquid if the temperature drops below the dew point inside the wall. In southern climates, the heat and moisture flow is generally reversed. A vapor retarder placed at the interior face of an exterior wall may cause moisture to condense between the vapor retarder and the wall. Foiled faced insulation, vinyl wall paper, and other products may act as vapor retarders. The condensed water may collect on the wall and/or floor below. Therefore, in southern climates a vapor retarder should generally not be placed at the interior face of an exterior wall.